This invention relates to a regulated power supply and in particular a regulated power supply for providing an output DC voltage of substantially constant level which depends on the magnitude of an input AC line voltage to a load irrespective of variations in load current.
A variety of DC voltage regulators are known in the art. Among these, a series-type voltage regulator is most widely used. Such a series-type voltage regulator comprises a rectifier circuit connected to receive an input AC voltage for generating an unregulated DC voltage, a control circuit connected between the rectifier circuit and a load and including at least one control semiconductor active device having a control electrode, a detector circuit for detecting variation in the regulated DC output voltage applied to the load, a reference voltage generating circuit, and a voltage comparator circuit adapted to compare the output of the detector circuit and the reference voltage of the reference voltage generating circuit and generate an error signal proportional to a difference between the output of the detector circuit and the reference voltage, thereby controlling the control circuit.
The series-type voltage regulator is capable of regulating an output DC voltage to a predetermined level irrespective of variations in the input AC voltage and load current. With variations in the inpt AC voltage in view, the voltage regulator should disadvantageously allow for a very great permissible dissipation in the control active device or transistor. Suppose, for example, that a variation width of a nominal 100 V input AC voltage is .+-.10%, and an unregulated output DC voltage of the rectifier circuit including a transformer is 50 V when the input AC voltage is 100 V and load current is zero ampere. Suppose further that, in this case, a regulated output DC voltage is 40 V, a load current is 0 to 5 A, and the internal resistance of the rectifier circuit is 1 ohm. Then, a maximum permissible dissiplation allowed for in the control transistor will become about 50 W when the input AC voltage is 110 V and load current is 5 A. This wattage is of the order of a magnitude required for output transistors in an audio stereophonic amplifier and this means that the control transistor requires, like the output transistor, a large-sized heat sink. The adaption of such regulated power supply circuit to an audio amplifier leads to a raise in cost of the audio amplifier. For this reason, the regulated power supply circuit is rarely employed in a commercially available amplifier. In order to improve a reproduction quality of the audio amplifier, however, it is preferable to use, as a power supply circuit for an output stage, a power supply circuit which involves no output voltage variation at least with respect to variations in load current, i.e., has a very small output impedance.